1. The Field of the Invention
The present invention relates generally to an acetabular cup implanted in a natural acetabulum and more particularly, the eccentric position of a socket within the acetabular cup and bevel of the anterior rim for use with a prosthetic femoral head to provide a more natural anatomic placement and function.
2. The Relevant Technology
When a primary total hip arthroplasty fails and requires surgical treatment with revision total hip arthoplasty, the previously implanted acetabular component is removed and a new acetabular cup implanted. Bone loss surrounding the acetabular component may also be present so that the revision acetabular cup is larger than the previously implanted primary acetabular cup. The revision (or jumbo) cup is therefore larger than the normal anatomic size of the acetabulum. Currently jumbo acetabular cups are used for acetabular cup revisions as well as for primary arthroplasty in patients with peri-acetabular bone defects, such as congenital hip dysplasia. Jumbo cups provide a large surface area for fixation to the bone and have better long term implant stability when using cementless fixation rather than cemented fixation. These jumbo cups restore the center of rotation of the hip more toward its anatomical location as compared to smaller cups that are placed into a superior bone defect above the anatomic acetabulum. Although current jumbo cups tend to have the socket of the cup centered within the metal shell, the cup is larger than the anatomic acetabulum which still causes the center of rotation of the hip joint to be higher than is ideal biomechanically.
Since current jumbo cups are larger than the anatomic dimensions of the native acetabulum, a portion of the rim of the cup often protrudes from the bone. This protrusion can cause impingement and irritation with the surrounding soft tissue such as the iliopsoas tendon, causing groin pain. This soft tissue impingement occurs most often on the anterior portion of the cup after being implanted into a natural acetabulum. However, soft tissue impingement can occur in more than just the anterior location.
Current jumbo cups are attached to the pelvis with screws placed through holes in the cup. Multiple screw holes are usually provided to permit variable placement of screws into the remaining peri-acetabular bone. Jumbo cups typically have screw holes in the peripheral portion of the cup which are inserted perpendicular to the face or rim of the cup. However, since the cup is larger than the normal acetabulum, the peripheral screws may be directed peripheral to the remaining peri-acetabular bone stock.
There is a need to have an acetabular cup that can provide a more natural anatomical center of rotation while allowing for greater fixation in the acetabulum. The cup should have the ability to use multiple cup liners or none at all. The cup should also reduce impingement on surrounding soft tissue.
As the above described techniques illustrate, the existing systems and procedures for acetabular cups may not be as effective as desired.